A variety of physiological conditions exist where irregular electrical activity of a heart results in an abnormal heartbeat. To correct the abnormal heartbeat, a multiplicity of systems and methods have been developed whereby at least one therapeutic electrical shock or pulse is applied to the heart through one or two electrodes to disrupt the irregular electrical activity so that the heart may revert to its regular electrical activity, resulting in a normal heartbeat.
Such systems and methods utilize electrical field stimulation wherein a sufficiently large shock is applied to the heart through one or two electrodes to produce a desired extracellular potential gradient in large regions of the heart distant from the electrodes. In this manner, the irregular electrical activity throughout the heart is sufficiently disrupted to permit restoration of normal electrical activity and normal heartbeat.
Many prior art systems and methods first detect, through a variety of known devices, a rhythmic condition such as tachycardia (relatively rapid heart action), arrhythmia (an alteration in the rhythm of the heartbeat either in time or force), tachyarrhythmia (arrhythmia characterized by a rapid, irregular heartbeat), and atrial or ventricular fibrillation (very rapid uncoordinated constrictions of the atria or ventricle, respectively, resulting in loss of the ability to pump blood, most particularly in the case of ventricular fibrillation). Next, an electrical shock is applied to the heart through one or two electrodes positioned on or within the heart. The electrical shock applied to the heart through the electrodes is large enough to produce an electric field in regions of the heart near and remote from the electrodes. Selected prior art systems include feedback mechanisms whereby the electrical condition of the heart is detected subsequent to the application of the first shock and, if a normal heart rhythm is not restored, an additional electrical shock is applied to the heart.
Measurements have recently been obtained utilizing optical mapping techniques with voltage-sensitive fluorescent dyes in hearts which indicate that the effects of an electrical stimulation pulse on the transmembrane potential of heart cells are greatest in the region within one millimeter of the stimulation electrode.
Because the strength of an electric field decreases with increasing distance from the electrodes, a pulse which produces an electric field sufficient for defibrillation in a region remote from an electrode will produce a much stronger electric field near the electrode. Injuries are produced in heart cells by such strong electrical stimulation when, during the electrical pulse, the transmembrane potential exceeds the threshold for cell membrane electroporation. As prior art cardioversion and defibrillation systems require the application of high voltage pulses to the heart to produce an electric field in regions of the heart remote from the electrodes, such systems also have large energy requirements.
One method, disclosed in U.S. Pat. No. 4,708,145 to Tacker, Jr. et al. attempts to avoid areas of high current density in the heart by using a sequential pulse, multiple pathway defibrillation method for controlling ventricular fibrillation and tachyarrhythmias. A catheter carrying a first and second electrode is located in the right ventricle and a third electrode is located either at the chest wall or at the abdominal cavity. The second and third electrodes are sequentially paired and pulsed with the first electrode to control ventricular fibrillation and other tachyarrhythmias. While the Tacker, Jr. et al device applies shocks to the heart through spaced apart electrodes, these shocks are intended to traverse the heart through particular pathways to establish a sufficient electric field in regions of the heart remote from the electrodes.
Since the effects of electrical shocks on transmembrane potentials are large near an electrode, a system to produce effects primarily in the cells near an electrode could require stimulation voltage or current that is less than the voltage or current needed to produce a desired electric field remote from an electrode. Such a system would require low current pulses which could decrease both the system energy requirements and cellular damage resulting from each electrical pulse. Low strength stimulation at a single site is already known to effect arrhythmias and even, as reported by Salama, halt ventricular tachycardia in isolated rapid hearts when applied to Purkinje fibers which occur on the endocardium.
Although the prior art provides a variety of cardioversion and defibrillation methods and systems, there remains a need for a method and system for cardioversion of a heart which utilizes low voltage pulses which minimizes damage to the heart with low power requirements.